In underground drilling, such as gas, oil or geothermal drilling, a bore is drilled through a formation deep in the earth. Such bores are formed by connecting a drill bit to sections of long pipe, referred to as a "drill pipe," so as to form an assembly commonly referred to as a "drill string" that extends from the surface to the bottom of the bore. The drill bit is rotated so that it advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. In any event, in order to lubricate the drill bit and flush cuttings from its path, piston operated pumps on the surface pump a high pressure fluid, referred to as "drilling mud," through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore.
The distal end of a drill string, which includes the drill bit, is referred to as the "bottom hole assembly." In "measurement while drilling" (MWD) applications, sensors (such as those sensing azimuth, inclination, and tool face) are incorporated in the bottom hole assembly to provide information concerning the direction of the drilling. In a steerable drill string, this information can be used to control the direction in which the drill bit advances.
Various approaches have been suggested for controlling the direction of the drill string as it forms the bore. The direction in which a rotating drill string is headed is dependent on the type of bit, speed of rotation, weight applied to the drill bit, configuration of the bottom hole assembly, and other factors. By varying one or several of these parameters a driller can steer a well to a target. With the wide spread acceptance of steerable systems in the 1980's a much higher level of control on the direction of the drill string was established. In the steerable system configuration a drilling motor with a bent flex coupling housing provided a natural bend angle to the drill string. The drill bit was rotated by the drilling motor but the drill string was not rotated. As long as the drill string was not rotated, the drill would tend to follow this natural bend angle. The exact hole direction was determined by a curvature calculation involving the bend angle and various touch points between the drill string and the hole. In this manner the bend angle could be oriented to any position and the curvature would be developed. If a straight hole was required both the drill string and the motor were operated which resulted in a straight but oversize hole.
There were several disadvantages to such non-rotating steerable drill strings. During those periods when the drill string is not rotating, the static coefficient of friction between the drill string and the borehole wall prevented steady application of weight to the drill bit. This resulted in a stick slip situation. In addition, the additional force required to push the non-rotating drill string forward caused reduced weight on the bit and drill string buckling problems. Also, the hole cleaned when the drill string is not rotating is not as good as that provided by a rotating drill string. And drilled holes tended to be tortuous.
Rotary steerable systems, where the drill bit can drill a controlled curved hole as the drill string is rotated, can overcome the disadvantages of conventional steerable systems since the drill string will slide easily through the hole and cuttings removal is facilitated.
Therefore it would also be desirable to provide a method and apparatus that permitted controlling the direction of a rotatable drill string.